arm, am33xx: add clk_get prototype

the clk_get() function is needed for the da8xx-fb video driver,
which is used on the am3xx based siemens boards.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>
Acked-by: Tom Rini <trini@ti.com>